Driver side airbag modules, which include an airbag cushion and an airbag module cover, are normally positioned within a hub of a steering wheel of an automobile. This happens to be the same area which conventionally includes the horn actuation switch. Accordingly, the airbag module cover must additionally serve to actuate the horn switch, preferably over a substantial area.
The introduction of the airbag module at the steering wheel hub complicated the design of the horn switch and the airbag module cover. The horn switch should not be susceptible to inadvertent actuation due to thermal expansion or contraction of the cover, curvature of the cover or close packing of the horn switch and a folded airbag cushion beneath the cover. In addition, the horn switch should not require a large degree of cover depression to be activated.
A number of horn switch designs have included a membrane type switch. Membrane switches conventionally comprise two very thin sheets having conductive coatings which are normally separated by thin spacers. Pressure on the switch pushes the conductive surfaces together to close a circuit and actuate the horn. The membrane switch is normally attached to the back of the airbag module cover which is depressed from the front by the driver to apply pressure to the horn switch. Various problems have been encountered in providing membrane switches that activate the horn in response to a force within a desired range, and membrane switches have the tendency to require an inconsistent amount of force to close.
In an attempt to overcome these problems, an existing horn switch assembly described in U.S. Pat. No. 5,309,135 uses a flexible potentiometer as a horn switch. The potentiometer is adhered to a flexible substrate which is adhered to the back of an airbag module cover. The resistance of the flexible potentiometer changes as it bends in response to a driver pressing against the module cover. A horn control circuit responds to extremely rapid changes in the resistance of the flexible potentiometer, but not to more gradual changes caused by, for example, temperature changes. However, because the potentiometer can bend or wrinkle in more than one axis or direction, the '135 horn switch assembly also requires an inconsistent amount of force to activate.
Some horn switch assemblies include a bend sensor. A bend sensor is similar to a potentiometer and includes a bend sensitive variable resistance circuit that is adhered to a flexible substrate. The resistance of the variable resistance circuit measurably changes as it is bent and the circuit is connectable to a horn control circuit that responds to rapid changes in resistance but not to more gradual changes. The variable resistance circuit works best when it is bent in only one axis or direction. However, existing horn switch assemblies incorporating a bend sensor have not been able to translate driver-applied force to the bend sensor so that the variable resistance circuit bends in only one direction. Because the circuit bends in more than one direction and wrinkles, the horn switch requires more driver-applied force to be activated. Additionally, these horn switch assemblies fail to translate a localized driver applied force to the cover over a greater area of the bend sensor.
In summary, providing a horn switch assembly that does not require an unreasonable degree of cover depression or an unreasonable amount of applied pressure for actuation, and is not susceptible to inadvertent actuation resulting from thermal expansion or contraction of the airbag module cover, curvature of the cover or close packing of the horn and a folded airbag cushion beneath the cover would be useful and desirable. More specifically, it would be preferably to provide a horn switch assembly utilizing a bend sensor wherein any localized applied force is distributed over a larger area of the bend sensor and the bend sensor is only allowed to bend in substantially one axis or direction.